Turbo equalization has typically been used in EDGE and LTE uplink to address inter-symbol interference (ISI) problem caused by channel dispersion. The solution has also been extended to multiple-input-multiple-output (MIMO) reception to address spatial-multiplexing interference in addition to ISI.
In a turbo interference cancellation receiver, interference such as inter-symbol-interference (ISI), code-division multiplexing (CDM) interference, and spatial-multiplexing interference due to single-user (SU) or multi-user (MU) MIMO can be cancelled based on soft estimates of the interfering symbols. The soft symbol estimates are formed using the decoder outputs, which describe the likelihood ratios of the bits that are used to determine these interfering symbols. Each likelihood ratio can be converted to a bit probability (i.e., probability of bit having value 0 or 1). After cancellation, the received signal is re-equalized using new combining weights, which reflect a new impairment covariance matrix due to interference cancellation. The equalized symbols are demodulated and converted to bit soft values, which are used by the various decoders, one for each user or MIMO stream, to produce updated bit likelihood ratios. Such an iterative multi-stage turbo equalization and interference cancellation receiver is referred to as a turbo-IC receiver.
All or almost all of the turbo-IC solutions developed in the prior art are intended for a time-division multiple-access (TDMA) system, e.g., EDGE, or a single-carrier frequency-division multiple-access (SC-FDMA) system, e.g., the LTE uplink. In a code-division multiple-access (CDMA) system, there are two issues. First, commercial CDMA systems such as IS-95, cdma2000, EV-DO, WCDMA, and HSPA all use symbol-dependent spreading waveforms. The spreading waveform of a symbol of interest is determined by a channelization code (e.g., orthogonal variable spreading factor (OVSF) code) and a pseudo-random scrambling code. Since the pseudo-random scrambling code has a period that is much longer than the symbol duration, the effective spreading waveform varies from symbol to symbol.
Thus, in a design of a turbo-IC receiver for a CDMA system, there is an issue of whether a code-specific design or a code-averaged design should be used. In the code-specific design, the time-varying aspects of the spreading waveform are accounted for. But in the code-averaged design, only the code statistics, averaged over a number of symbol intervals, are used in the design. Such a design choice can be made with regard to equalization weights (chip-level or symbol-level, explained further below), interference cancellation, etc.
Another issue with the CDMA system is that the receiver processing for a CDMA signal can happen in the chip domain, i.e., before despreading, or in the symbol domain, i.e., after despreading. For example, equalization can be applied in the chip domain such as in a chip equalization receiver, or in the symbol domain such as in a generalized Rake (G-Rake) receiver.